1. Field of the Invention
The present invention relates in general to a method of controlling the shape of a strip rolled by a rolling mill, and more particularly to a method suitable for precisely controlling the shape of the rolled strip in its width direction.
2. Discussion of the Prior Art
A rolling mill having four or six rolls is known for rolling an aluminum strip or other metal strips. To avoid defects or shape errors so-called "edge wave" along the edges of the rolled strip and "center buckling" in a middle portion of the rolled strip as viewed in the direction of width of the strip, there have been used various shape correcting devices such as: press-down or roll tilt adjusting device for tilting a pair of work rolls of the mill; work roll bending force adjusting device for adjusting the bending force applied to the work rolls; bending force differential adjusting device for adjusting a difference between the bending force values as measured at both ends of the rolls; and intermediate roll bending force adjusting device for adjusting the bending force applied to intermediate rolls between which the work rolls are disposed. These shape correcting devices function to make appropriate corrections to eliminate the shape errors of the rolled strip.
An example of a strip shape control system for a thin-strip rolling mill is disclosed in Publication 1-50485 (1989) of examined Japanese Patent Application. This strip shape control system includes a shape sensor having a plurality of sensing elements disposed at different positions in the direction of width of the strip, and providing output signals which collectively represent the strip shape. The strip shape control system further includes a plurality of shape correcting devices such as: bending mechanisms for bending the rolls in the horizontal plane; bending mechanisms (usually referred to as "jacks") for bending the rolls in the vertical plane, and press-down or tilting mechanism (usually referred to as "screws") for tilting the rolls in the vertical plane. A controller capable of performing arithmetic operations is provided to obtain the shape distribution of the rolled strip detected by the shape sensor, and a calculated desired shape distribution of the strip, as functions of the transverse position in the width direction of the strip. Further, the detected shape distribution is obtained as a function of the transverse position of the strip, with respect to the unit operation amount of each shape correcting device. Based on these functions obtained, the controller calculates an evaluating function for evaluating the shape of the rolled strip over the entire width of the strip, and calculates the operation amounts of the shape correcting devices that minimize the evaluating function, so that the shape correcting devices are activated by the calculated operation amounts, to control the transverse shape distribution of the rolled strip.
However, the known shape control system or method for a thin-strip rolling mill indicated above is susceptible to an influence of a delay in the detection of the strip shape distribution by the shape sensor, which inevitably results in delayed response of the adjusting actuators of the roll bending and tilting mechanisms ("jacks" and "screws") due to the delayed detection by the shape sensor. Accordingly, the known strip shape control method suffers from delayed control of the strip shape in response to the detected output of the shape sensor, leading to potential difficulty in assuring sufficiently high precision of the strip shape control. Further, since the method in question does not utilize a detected change in the rolling force, the method has a tendency of low response to the strip shape variation due to the change in the rolling force, which occurs when the rolling speed is changed. This problem is serious particularly in the case of rolling of an aluminum strip.